Well-defined nanomagnetic nitrilotriacetic acid complex of Cu(ii) supported on silica-coated nanosized magnetite: a new highly efficient and magnetically separable catalyst for C–N bond formation

A nitrilotriacetic acid (NTA) complex of Cu(ii) supported on silica-coated nanosized magnetite Fe3O4@SiO2-Pr-DEA-[NTA-Cu(ii)]2 was prepared as a new well-defined magnetically separable nanomaterial and fully characterized via IR, XRD, FESEM, TEM, TGA, DLS, BET, VSM, solid-state UV-vis spectroscopy, EDX, ICP-OES, and FESEM-EDX map analyses. Thereafter, it was successfully applied as a new easily magnetically separable and reusable heterogeneous nanocatalyst for the Buchwald–Hartwig C–N bond formation reaction in DMF at 110 °C. Using this method, various kinds of nitrogen heterocycles, such as imidazoles, 2-methyl-1H-imidazole, benzimidazole, indole, and 10H-phenothiazine as well as aliphatic secondary amines such as piperidine, piperazine, morpholine, dimethylamine, and diethylamine, were reacted with aryl halide compounds, and the desired products were obtained with good to excellent yields. In all cases, the applied catalyst could be recovered easily and rapidly using an external magnet and reused 7 times without significant loss of catalytic activity.


Introduction
Considering the basic principles of green chemistry, 1 a suitable catalyst exhibits the highest efficiency even at the lowest concentration and can be easily separated and reused. 2 Because most of the chemical reactions take place on the surface of catalysts, catalysts with a higher surface-to-volume ratio will be more efficient at accelerating reactions.Accordingly, nanocatalysts that provide high surface-to-volume ratios are highly suitable as green catalysts because using them in very small quantities increases economic efficiency and prevents the release of hazardous substances into the environment.Moreover, there are other benets, such as the manipulation of size, shape, composition, and morphology. 3All these advantages lead to the widespread application of nanocatalysts in several prominent industrial chemical processes, such as water purication, 4 biodiesel production, 5 photocatalysis, 6 and drug delivery. 7Nevertheless, one of the most important problems in using nanocatalysts is the difficulty in separation due to the very small size of nanoparticles (NPs) and the need for special methods, such as ultraltration and high-speed centrifugation.Unfortunately, these methods are costly, involve time and energy, and are mostly inefficient, thus contradicting the green chemistry principles.One of the preferred strategies to overcome this problem is to magnetize nanocatalysts.Magnetic nanocatalysts can be easily and efficiently removed from reaction mixtures using an external magnetic eld.Due to this special property, magnetic nanoparticles (MNPs), such as magnetite (d-Fe 3 O 4 ) and maghemite (Y-Fe 2 O 3 ), have attracted much attention for use as substrates and catalysts among other iron oxide nanomaterials, leading to interesting ndings. 8Magnetite (d-Fe 3 O 4 ) NPs with a wide range of advantages, such as tunable and very small particle sizes, high magnetic permeability, good stability, low cost, ease of preparation, and reasonably large specic surfaces, are the most important and popularly used MNPs in the design and preparation of novel magnetically separable nanocatalysts. 9Nevertheless, their high free surface energy and anisotropic dipolar attraction lead to the facile and rapid aggregation of d-Fe 3 O 4 NPs, resulting in the reduction of surface area and catalytic activity.A frequently used strategy to prevent the aggregation of d-Fe 3 O 4 NPs is coating them with SiO 2 , which provides a core-shell system. 10he Fe 3 O 4 @SiO 2 core-shell system simultaneously provides the magnetic behaviour of d-Fe 3 O 4 and the tunable surface of SiO 2 NPs, thus efficiently supporting the design and preparation of various types of magnetically separable heterogeneous nanocatalysts.
Arylamines and heteroarylamines are important precursors used for the synthesis of drugs, agrochemicals, and a wide range of natural products, 11 complex molecules, such as dendrimers 12 and polymers, 13 dyes and pigments, 14 and molecules with nonlinear optical features. 15ver since Buchwald and Hartwig introduced palladiummediated amination of aryl halides for the preparation of arylamine derivatives, 16 many of the conditions have been improved, making the Buchwald-Hartwig method an extremely useful and synthetically vital technique.The lower reaction temperature, a wide range of available substrates, greater selectivity toward amines, better functional group compatibility, and the lack of formation of highly reactive species 17 are the main advantages of the Buchwald-Hartwig method over other C-N bond formation strategies, such as nucleophilic aromatic substitution, Ullmann coupling, and nitration followed by reduction. 18However, the application of palladium, which is a very expensive and toxic catalyst, is an important drawback that overshadows the general use of the Buchwald-Hartwig method.Efforts to solve this problem have led to the use of copper as a more economical metal with vast abundance and consequently, low cost. 19Nevertheless, the currently reported methodologies suffer from numerous drawbacks, such as long reaction times, high reaction temperatures, the need for stoichiometric amounts of copper reagents, the use of toxic and air-sensitive ligands, and also very low yields; excessive amounts of aryl halides or amines are required to achieve reasonable product yields. 20Thus, the design and preparation of novel Cu-based catalytic systems to overcome these drawbacks are of great interest.
Considering all the advantages of magnetically separable nanocatalysts and also the interesting features of single-atom catalysts, which provide unique opportunities for the design of effective, selective, and stable heterogeneous catalysts with well-dened active centres for a wide variety of chemical reactions, 21 we introduce the NTA complex of Cu(II) supported on silica-coated nanosized magnetite (Fe 3 O 4 @SiO 2 -Pr-DEA-[NTA-Cu(II)] 2 ) (4) as a novel well-dened nanomagnetic catalyst for the Buchwald-Hartwig C-N bond formation reaction (Scheme 1).

Experimental methods
All chemicals were obtained from Sigma and Fluka and used as received without further purication.All the solvents were distilled and dried before use.The progress of the reactions was followed by TLC using silica gel polygrams SIL G/UV 254 plates.The eluent solvent used was petroleum ether, ethyl acetate, or a mixture of both.All yields refer to isolated products aer indicated purication methods.The products were characterized by spectral data analysis.Their melting points were determined in open capillary tubes using a Büchi B-545 melting point apparatus.The inductively coupled plasma (ICP) analysis was carried out using an ICP analyzer (Varian, Vista-Pro).Elemental analyses were performed on a Thermo Finnigan CHNS analyzer (1112 series).The X-ray diffraction (XRD) patterns of the catalysts were recorded by a Bruker AXS D8advance X-ray diffractometer using CuKa radiation (l = 1.54178Å) in 2q scanning range of 20°to 90°.The samples were characterized by Field-Emission Scanning Electron Microscopy (FESEM-FEI, TESCAN, model MAIA3).Transmission electron microscopy (TEM) images were recorded using a Zeiss instrument operated at an accelerating voltage of 100 kV.The Brunauer-Emmett-Teller (BET) surface area analysis was adopted at 77 K to obtain the surface areas using a Belsorp mini II apparatus (Microtrac Bel Corp, Japan) aer the samples were degassed using a ow of N 2 .To evaluate the thermal decomposition of the individual components and their mixture, Thermogravimetric Analysis (TGA) was performed on a Perki-nElmer device manufactured by Thermal Sciences.O (0.9 g, 4.5 mmol) were added to a surfactant solution of polyvinyl alcohol with a mean molecular weight of 15 000 Da (1 g) in water (30 mL), and the obtained mixture was stirred vigorously with a mechanical stirrer.The mixture was heated to 80 °C and then an appropriate amount (1.0 mole per liter of solution) of hexamethylenetetramine (HMTA) was added dropwise until the reaction media reached pH 10.The black magnetic particles of Fe 3 O 4 formed were collected using an external magnet, washed with ethanol (50 mL, 3 times) and deionized water (50 mL, 3 times), and dried at 80 °C for 10 h.
Preparation of Fe 3 O 4 @SiO 2 NPs (5) The Fe 3 O 4 @SiO 2 NPs were prepared by a modied Stober method. 22In a typical procedure, the prepared Fe 3 O 4 particles (0.5 g) were dispersed in a mixture of ethanol (50 mL), deionized water (5 mL), and tetraethoxysilane (TEOS) (0.188 g, 0.20 mL).Aerward, 5 mL of NaOH (10 wt%) was added dropwise.Aer stirring for 30 min at room temperature, the magnetic nanoparticles were collected using an external magnet and washed with ethanol (50 mL, 3 times) and deionized water (50 mL, 3 times) and dried at 80 °C for further uses.
Preparation of N-propylchloride-functionalized silica-coated magnetite nanoparticles (Fe 3 O 4 @SiO 2 -Pr-Cl) (7) To a 50 mL round-bottom ask containing 20 mL of ethanol, Fe 3 O 4 @SiO 2 (0.6 g) was added and thoroughly dispersed under ultrasonic irradiation.Aer this, 3-chloromethoxypropylsilane (3 mmol, 0.4 g, 0.5 mL) was added, and the resulting mixture was reuxed for 12 h.Aer that, all the insoluble species were collected using an external magnet, washed with ethanol (50 mL, 3 times) and water (50 mL, 3 times), and dried at 80 °C for 12 h, and Fe 3 O 4 @SiO 2 -Pr-Cl nanoparticles were obtained as a brown powder. 23eparation of 2,2 0 -((3-propyl)azanediyl)bis(ethan-1-ol)functionalized silica-coated MNPs (Fe 3 O 4 @SiO 2 -Pr-DEA) (9) To a 50 mL round-bottom ask containing a solution of N,Ndiisopropylethylamine (3 mmol, 0.38 g, 0.4 mL) in absolute ethanol (20 mL), diethanolamine (DEA) (3 mmol, 0.315 g, 0.3 mL) was added, and the solution was stirred at ambient temperature for 2 h.Aer this, Fe 3 O 4 @SiO 2 -Pr-Cl nanoparticles (1 g) were carefully added to the solution, and the obtained mixture was stirred vigorously for 12 h under reux conditions.The Fe 3 O 4 @SiO 2 -Pr-DEA MNPs formed a brown powder, which was magnetically isolated from the crude using an external magnet.The dried magnetic nanoparticles were obtained aer washing the obtained powder with ethanol (50 mL, 3 times) and water (50 mL, 3 times) and heating in an oven at 80 °C for 6 h.
The resulting mixture was reuxed for 6 h.Then, the reaction mixture was cooled to room temperature, and the insoluble materials were separated using an external magnet, washed with water (5 mL, 2 times) and ethanol (5 mL, 2 times), and Fe 3 O 4 @SiO 2 -Pr-DEA-[NTA-Cu(II)] 2 was obtained as a grey powder aer drying at 60 °C under reduced pressure.
General procedure of the Buchwald-Hartwig C-N bond formation reaction between aryl halides (1) and N-containing compounds (2) in the presence of Fe 3 O 4 @SiO 2 -Pr-DEA-[NTA-Cu(II)] 2 (4) A mixture of N-containing compound 2 (1.2 mmol), aryl halide 1 (1 mmol), Cs 2 CO 3 (2 mmol, 0.65 g), and DMF (3 mL) was stirred in the presence of nanomagnetic catalyst 4 (0.05 g) at 110 °C, and the progress of the reaction was monitored by TLC.Aer completion of the reaction, the catalyst was separated using an external magnet, washed with EtOH (5 mL, 2 times), dried at 70 °C for 12 h, and reused.The residual solution was immediately poured into saturated brine (10 mL) and extracted with EtOAc (10 mL, 3 times).The combined organic layer was dried over anhydrous Na 2 SO 4 and concentrated under reduced pressure.The residue was puried by column chromatography on silica gel using a petroleum ether/ethyl acetate (5 : 1 v/v) mixture as the eluent to afford the pure product.The chemical properties and physical structure of the synthesized nanocatalyst were studied by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction analysis (XRD), dynamic light scattering (DLS), eld-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), inductively coupled plasma (ICP), vibrating sample magnetometer (VSM), energy dispersive X-ray analysis (EDX) and ultraviolet-visible spectroscopy (UV-vis).

Results and discussion
The FT-IR spectra of the synthesized magnetite NPs are presented in Fig. 1.In the FT-IR spectrum of magnetite NPs (Fig. 1, curve a), peaks were found at 584 and 3384 cm −1 corresponding to the stretching vibrations of Fe-O and the hydroxyl groups of Fe 3 O 4 NPs, respectively.The presence of Fe-O stretching vibration in all the other spectra claries the existence and stability of the magnetite NPs in all the other NMPs prepared (Fig. 1, curves b, c, d, e, and f).In the FT-IR spectrum of Fe 3 O 4 @SiO 2 (5), the peaks at 1026 and 1088 cm −1 belong to the Si-O stretching vibrations (Fig. 1, curve b).The stretching vibrations of Si-O bonds were also observed in the FT-IR spectra of 7, 9, 11 and 4 NPs at 1025 and 1084, 1092, 1104, and 1049 cm −1 , respectively (Fig. 1, curves c, d, e, and f).The FT-IR spectrum of 7 contained the C-Cl stretching, CH 2 bending and sp 3 C-H stretching vibration peaks at 687, 1425, and 2905 cm −1 , respectively, as expected (Fig. 1, curve c).The     5a-c, respectively.Notably, in a superparamagnetic material, without any external magnetic eld (H = 0), the magnetic vectors of each magnetic particle are randomly placed in different directions and their total result is zero. 27As shown in Fig. 5, in the VSM curves of all three samples examined, no hysteresis loop or remanence (M r300K = 0) was detected at 300 K.Moreover, the coercivity value was zero (H C300K = 0) for all samples; these data suggest the superparamagnetic behaviour of the studied samples.The saturation magnetization values were found to be 70.), which can be related to the desorption of water vapor and other volatile organic compounds adsorbed on the catalyst and the loss of covalently bonded organic groups, respectively (Fig. 6a).Based on the results obtained from the thermogram, the content of organic moieties was about 28.7% against the solid support and other inorganic materials.Considering the data obtained from TGA, the loading organic ligand was calculated to be approximately 0.59 mmol per gram of synthesized catalyst.Moreover, DSC analysis was carried out in the range of 50-800 °C under an N 2 atmosphere at 10 °C min −1 , which showed two endothermic peaks at 124.09 and 272.73 °C (Fig. 6b).The results of DSC conrm the TGA results of Fe 3 O 4 @SiO 2 -Pr-DEA-[NTA-Cu(II)] 2 NPs.
ICP-OES was used for the determination of copper content in the freshly synthesized catalyst.Using this method, the actual Cu content of Fe 3 O 4 @SiO 2 -Pr-DEA-[NTA-Cu(II)] 2 NPs was measured to be 1.28 mmol of Cu per gram of the catalyst, which is in good agreement with the data obtained from the TG analysis and the organic ligand content measured in the catalyst.
The existence of Cu(II) in the chemical structure of Fe 3 -O 4 @SiO 2 -Pr-DEA-[NTA-Cu(II)] 2 NPs was also investigated by solid-state UV-vis spectroscopy, and the obtained spectra of Fe 3 O 4 NPs, Cu(OAc) 2, and freshly synthesized Fe 3 O 4 @SiO 2 -Pr-DEA-[NTA-Cu(II)] 2 NPs are presented in Fig. 9a-c, respectively.In the UV-vis spectrum of Fe 3 O 4 @SiO 2 -Pr-DEA-[NTA-Cu(II)] 2 NPs, the absorption bands at around 295 and 367 nm are representative of the Cu 2+ species. 28he Brunauer-Emmett-Teller (BET) analysis was used to study the porous structure and surface area of freshly synthesized Fe 3 O 4 @SiO 2 -Pr-DEA-[NTA-Cu(II)] 2 NPs, and the obtained results are summarized in Table 1 and Fig. 10.The measured specic surface area was 90.21 m 2 g −1 with a total pore volume of 0.1488 cm 3 g −1 and a mean pore diameter of 6.5994 nm.
Aer the successful preparation and characterization of Fe  catalyst, its catalytic activity in the Buchwald-Hartwig C-N bond formation reaction was studied (Scheme 1).For this, the reaction of imidazole (2a, 1.2 mmol) and iodobenzene (1a, 1 mmol) was selected as the model reaction (Scheme 1), and the time and the yield of the reaction were monitored under different conditions, such as solvent, temperature, base and the amount of catalyst, and the obtained results are summarized in Table 2.
Based on the obtained results, it is obvious that the base, solvent, and catalyst play crucial roles in promoting the studied reaction.As seen in Table 2, moderate to good yields were obtained in DMF, DMSO, NMP, MeCN, and toluene (Table 2, entries 1, 2, 4, 8, and 9), while the reactions carried out in DME, H 2 O, EtOH, and MeOH were not effective, and very low yields were obtained aer a long time (12 h) (Table 2, entries 3, 5, 6 and 7).The best results were obtained when DMF was used as the solvent (Table 2, entry 1).In addition, the reaction temperature directly affected the yield and time of the reaction.The shortest reaction time (1.5 h) and the best reaction yield (93%) were obtained at 110 °C (Table 1, entry 1).
Increasing the temperature up to 140 °C had no signicant effect on the time and yield of the reaction (Table 2, entries 10, 11, and 12).However, lowering the temperature signicantly increased the reaction time and decreased the yield (Table 2, entries 13 and 14).
As the results indicate, the reaction was highly sensitive to the presence of the catalyst and did not proceed without the catalyst even aer a long time (12 h) (Table 2, entry 23).The best results were obtained using 0.05 g of the catalyst (Table 2, entry 1), and increasing the catalyst quantity did not have   Paper RSC Advances a signicant effect on the reaction time and yield (Table 2, entries 15 and 16), whereas, a decrease in the quantity of catalyst led to a signicant increase in reaction time and decrease in yield (Table 2, entries 17 and 18).The effect of different bases, including Cs 2 CO 3 (Table 2, entry 1), NaOH (Table 2, entry 19), K 3 PO 4 (Table 2, entry 20), NaOAc (Table 2, entry 21), and K 2 CO 3 (Table 2, entry 22), on the reaction was also studied, and the best results were obtained with Cs 2 CO 3 (Table 2, entry 1).Therefore, considering all these results, the best reaction conditions for the reaction of iodobenzene (1a, 1 mmol) and imidazole (2a, 1.2 mmol) in the presence of Fe 3 O 4 @SiO 2 -Pr-DEA-[NTA-Cu(II)] 2 were: DMF (3 mL) as the solvent, 0.05 gram of the catalyst, Cs 2 CO 3 (2 mmol, 0.65 g) as the base and a reaction temperature of 110 °C.Under the optimized reaction conditions, the versatility and generality of Fe 3 O 4 @SiO 2 -Pr-DEA-[NTA-Cu(II)] 2 as a new nanomagnetic nanocatalyst were investigated, and the obtained results are summarized in Table 3.As shown in Table 3, various kinds of nitrogen heterocycles, such as imidazole (Table 3, entries 1-6), 2-methyl-1H-imidazole (Table 3, entries 7, 8, and 9), benzimidazole (Table 3, entries 10, 11, and 12), indole (Table 3, entries 13, 14, 15 and 16) and 10H-phenothiazine (Table 3 entries 17, 18 and 19), were applied successfully, and the desired products were obtained with good to excellent yields.Moreover, some secondary aliphatic amines, such as piperidine (Table 3, entries 20, 21, and 22), piperazine (Table 3, entry 23), morpholine (Table 3, entries 24 and 25), dimethylamine (Table 3, entries 26 and 27) and diethylamine (Table 3, entry 28), were tested.And the desired products were formed with good yields.The effect of aryl halide on the efficiency of the developed method was also investigated.The above-mentioned NHcontaining compounds were reacted with aryl iodide, aryl bromide, and aryl chloride in the presence of NTAC-Cu(II)-SSCNM NPs under optimized reaction conditions, and the obtained results are summarized in Table 3.As seen in Table 3, the efficiency of the as-developed method is highly sensitive to C-X (X: halide) bond reactivity; a decrease in reaction yield and an increase in reaction time were observed with the change of halide from iodide to bromide and chloride.With aryl iodides, the reactions occurred in relatively shorter durations (1-8 h), and the desired products were obtained with good to excellent yields (80-95%).With aryl bromide, although the reaction time increased (3-15 h), the yield of desired products was still good (80-92%).However, with aryl chloride, the reaction yields were greatly reduced; even aer a very long time (24 h), only small amounts of desired products were obtained (24-40%).Another factor that affects the reaction time and yield is the presence of electron-withdrawing and -donating substituents on the aromatic ring of the applied aryl halides.As seen from the results in Table 3, the presence of electron-withdrawing groups on the aromatic rings of aryl halides decreased the reaction time and increased the reaction yield (Table 3, entries 2, 8, 10 and 14), while aryl halides with electron donor groups presented increased reaction time and decreased reaction yields (Table 3, entries 3, 11, 12, 15, 16 and 19).
A plausible mechanistic pathway is proposed for the Buchwald-Hartwig C-N bond formation reaction of aryl halides (a) and s-amines (b) in the presence of NTAC-Cu(II)-SSCNM NPs, which serve as a new, highly efficient and magnetically separable nanocatalyst (Scheme 4). 29,30As shown in Scheme 4, the catalytic cycle starts with the in situ generation of Cu(I) from Fe 3 O 4 @SiO 2 -Pr-DEA-[NTA-Cu(II)] 2 NPs under the reaction conditions.Aer this, the transient Cu(III) species (A) is produced through oxidative addition, followed by the addition of the NH compound, which leads to the formation of intermediate (B).Then, the reductive elimination process from (C) produces the C-N bond product.Finally, the Cu(I) species is reoxidized to Cu(II) in the presence of air, 31 and the catalytic cycle continues in the same way until the end of the reaction.
Considering the principles of green chemistry, facile separation and reusability are among the essential requirements of an efficient and environmentally friendly catalyst.The reusability of Fe 3 O 4 @SiO 2 -Pr-DEA-[NTA-Cu(II)] 2 NPs as a highly efficient and nanomagnetic catalyst was studied in the model reaction of imidazole (2a) and iodobenzene (1a) under the optimized reaction conditions.Aer the completion of the b Isolated yields.
Table 3 The The chemical and physical stabilities of the catalyst were investigated aer the 7th reuse cycle by IR, XRD, FESEM, DLS, and ICP-OES analyses (Fig. 12).The IR of the recovered catalyst aer the 7th reuse cycle (Fig. 12a) was completely the same as the freshly synthesized catalyst (Fig. 1f), indicating the stability of the chemical structure of the catalyst.Moreover, there was no difference between the XRD of the freshly synthesized (Fig. 2c) and recovered catalysts aer the 7th catalytic cycle (Fig. 12b), and the mean size of the Fe 3 O 4 nanocores was calculated using the Debye-Scherrer equation to be around 15 nm, which is approximately the same as that of the freshly synthesized catalyst.
The FESEM image and DLS results of the recovered catalyst aer the 7th reuse cycle are presented in Fig. 12c and d, respectively.In the SEM image, signicant changes were not observed in the surface morphology of the catalyst, and the recovered nanoparticles showed approximately spherical shapes (Fig. 12c).The DLS analysis results indicate that the size distribution was mostly between 47-49 nm, which is larger than the size of the freshly prepared nanocatalyst and may be the factor responsible for the smooth decrease in the catalytic activity of the recovered catalyst from the 5th reuse cycle.The Cu content of the recovered catalyst aer the 7th reuse cycle was investigated by ICP-OES analysis and was determined to be 1.275 mmol g −1 , indicating the stability of Cu in the structure of the catalyst and the lack of leaching.Moreover, the possibility of Cu leaching from the surface of the nanocatalyst was also investigated by the hot ltration test aer the model reaction of iodobenzene (1a) and imidazole (2a).
Aer 30 min of the reaction, the catalyst NPs were removed from the reaction mixture, and the progress of the reaction was checked in the residue.The results are demonstrated in Fig. 13.As seen in Fig. 13, the reaction stopped aer the removal of the catalyst NPs, indicating that no catalytically active copper species were present in the residue solution.In addition, the existence of Cu species in the reaction solution was checked aer the completion of the reaction by ICP-OES.For this purpose, the model reaction was conducted under optimized conditions, and aer the completion of the reaction, the catalyst was separated, and the solvent was evaporated under reduced pressure.The residue was dissolved in HNO 3 and subjected to ICP-OES analysis.The results showed that copper did not exist in the analysed samples.Both ndings indicate that copper species did not leach from the surface of Fe 3 O 4 @SiO 2 -Pr-DEA-[NTA-Cu(II)] 2 NPs into the solution, and the reactions were heterogeneously catalyzed.
Finally, to evaluate the efficiency of Fe 3 O 4 @SiO 2 -Pr-DEA-[NTA-Cu(II)] 2 NPs as a new, highly efficient, magnetically separable and reusable nanocatalyst, its activity in the Buchwald-Hartwig C-N bond formation reaction of iodobenzene (1a) and imidazole (2a) was compared with some other catalysts that have been reported previously.The data listed in Table 4 show that Fe 3 O 4 @SiO 2 -Pr-DEA-[NTA-Cu(II)] 2 performs the reaction in a shorter duration using a lower amount of copper and produces the desired product (3a) with excellent yield.Another important advantage of the as-prepared nanomagnetic catalyst is its facile separability from the reaction media and reusability.
Buchwald-Hartwig C-N bond formation reaction of aryl halides and NH-containing compounds in the presence of Fe 3 O 4 @-SiO 2 -Pr-DEA-[NTA-Cu(II)] 2 NPs a Entry X Y Product Time (h) Yield d (catalyst was simply separated using an external magnet, washed with ethanol, and reused in the next run aer drying at 70 °C for 12 h.The model reaction could be run seven times with the recovered Fe 3 O 4 @SiO 2 -Pr-DEA-[NTA-Cu(II)] 2 NTAC-Cu(II)-SSCNM NPs in each run without any considerable loss of catalytic activity (Fig. 11).

Table 2
Optimization of the reaction parameters for the production of 1-phenylimidazole (3a) in the presence of Fe 3 O 4 @SiO 2 -Pr-DEA-[NTA-Cu(II)] 2 NPs a

Table 4
The comparison of the catalytic activity of Fe 3 O 4 @SiO 2 -Pr-DEA-[NTA-Cu(II)] 2 NPs with some catalysts reported for the Buchwald-Hartwig C-N bond formation reaction between iodobenzene and imidazole a Isolated yield.